Security elements within the terms of the invention are, for example, security threads and security strips for bank notes and for other documents of value, tear threads for packaging, labels and tags, which are suitable for detecting the authenticity of an object connected with them, in particular, document of value. Documents of value within the terms of the invention can be bank notes, identity cards, checks, passports, travel tickets, admission tickets and the like. But the invention is also suitable for the protection of any other objects of value and their packaging, such as e.g. books, CDs and the like.
Multilayer security elements in the form of security threads find a wide range of use in bank notes. They comprise at least one carrier substrate—normally formed as a transparent plastic film—to which the further layers are applied. These further layers mainly are printed on or, in particular in the case of metallic layers, vapor-deposited, but they can also be, for example, sputtered or sprayed.
Not always are all layers applied all-over. They can be disposed side-by-side and/or one above the other. They can form characters or patterns or can have gaps, either from the outset or as a result of a subsequent partial removal of material, so as to form, for example, a negative writing, which when viewed in incident light is hardly visible, but when viewed in transmitted light produces a clearly visible contrast which is due to the transparency of the carrier substrate. Furthermore, the layers can have optically variable effects and for that purpose can have, in particular, diffraction structures in the form of grating patterns or holograms etc. The layers can also have machine readable security features, such as e.g. electrical conductivity in the case of continuous metallic coatings or in the case of printed layers doped with electrically conductive particles. Additionally or alternatively, they can have magnetic properties and/or luminescent properties, in particular in areas not visible luminescent substances are often used. The machine readable security features can also be formed as locally restricted machine readable code, for example bar code.
A basic matter of concern when producing such multilayer security elements is to achieve a side-independent appearance, so that when fitting the security elements to or embedding them in the objects to be such secured no particular measures become necessary for their application with regard to trueness to side. This problem in particular arises when incorporating security threads as so-called window threads into papers of value, in particular bank notes, since these threads and strips tend to twist.
In the case of a simple metalized security thread with hidden magnetic layer a side-independent appearance is easy to achieve, when, for example, at first a carrier substrate is printed with the magnetic layer and subsequently is metalized all-over on both sides. In this connection it is also known, that firstly a foil is metalized, the magnetic layer is applied onto the metal layer, and then the foil is cut, superimposed and adhesively bonded in a roller laminating plant in such a way as to produce a thread-shaped foil compound with two outside foils, two inside metal layers and one central, i.e. located between the metal layers, double magnetic layer (EP 0 374 763 A2). By means of the two outside foils the coatings are protected against outside influences. Furthermore, the absolutely symmetrical layer structure of the foil compound prevents a curling of the produced thread which would lead to the formation of garlands.
This laminating method, however, is not suitable for complex layer structures, wherein different layers are positioned locally restricted at different places which are disposed in an exact relation to each other. Since the cutting and superimposing of the individual compound foils inevitably leads to the fact, that the different, locally restricted layers will not be disposed exactly regular to each other in the final layer compound.
Therefore, complex layer structures are built on one single carrier material. For example, in WO 92/11142 are described several variants of a security thread with hidden magnetic layer or hidden magnetic code and integrated negative writing.
In the simplest case the negative writing is identically produced in the magnetic layer and in two metal layers which cover the magnetic layer. For this purpose at first an activable printing ink is applied in a usual fashion to the area of the future negative writing on a transparent plastic film. Then a first metal layer is vapor-deposited thereon and a magnetic layer is printed all-over on top of it, the latter then is covered with a second vapor-deposited metal layer. The following activation of the printing ink results in congruent gaps in the three layers located on top of the printing ink. The inner metal coating ensures, that the security thread has, due to the transparency of the carrier foil, the same appearance independent of side.
In case of complex layer structures, in which the negative writing is not produced congruent in all layers, however, it is difficult, to achieve an appearance which is exactly the same from both viewing sides.
Therefore, in case of such complex layer structures at least one, optionally also the two metallic layers are printed in register onto the desired areas (WO 92/11142). The problem here is that the metallically appearing printing inks, e.g. supersilver, are less brilliant than vapor-deposited metal layers and that supersilver does not have a good electrical conductivity. In the event that one of the two metallic layers is a printed layer and the other one is a real, e.g. vapor-deposited metal layer, an exactly side-independent appearance will not be achieved. In the other event, that both metallically appearing layers are printed layers, the optical appearance indeed will be identically in a side-independent fashion, but in total will not be as brilliant as one would desire and also not electrically conductive.